An electric wire insulated with a foamed plastic, which has a foamed fluoroplastic as an insulating layer is widely used as plenum coaxial cables or signal transmitting electric cables used in electronic equipment. It is known that a fluoroplastic is excellent in flame retardancy, heat resistance, electrical properties, mechanical properties, resistance to chemicals, etc. Further, when the fluoroplastic is foamed, its dielectric constant is lowered. It follows that the above insulated electric wire can shorten the signal transmission delay time, improving the signal transmission speed.
The signal processing capacity of an electric wire insulated with a foamed plastic which used a fluoroplastic is on a sharp increase in recent years. As a result, it is of high importance to diminish the nonuniformity in the signal transmission time. Alternatively, it is required that the signal transmission speed be further improved. To meet these requirements, it is necessary to improve the expansion ratio of the foamed insulating layer of a fluoroplastic and to diminish the nonuniformity in the expansion ratio so as to stabilize the outer diameter of the insulated electric wire. Also, when the insulated wire is used as inner wiring in, for example, a computer, it is required that the thickness of the insulating layer be further diminished and that the diameter of the insulated wire be further diminished in accordance with miniaturization and increased in capacity of the apparatus.
In the conventional methods of manufacturing an electric wire insulated with a foamed plastic, a fluorine-based foaming agent such as a chlorofluoro carbon (CCl.sub.3 F, CCl.sub.2 F-CClF.sub.2, CClF.sub.2 -CF.sub.3) or hydrochlorofluoro carbon (CHClF.sub.2) is introduced into a fluororesin in a molten state. Then, the resin composition containing the foaming agent is extruded to cover a conductor wire, followed by foaming the resin composition so as to manufacture the desired electric wire insulated with a foamed plastic.
However, the fluorine-based foaming agent used in the conventional method contains chlorine doing damage to the ozone layer of the earth. It follows that using such a foaming agent causes an environmental pollution problem.
Also, in the case where an electric wire insulated with a foamed plastic is manufactured by using the fluorine-based foaming agent, the expansion ratio of the insulating layer which has a thickness of at least 0.5 mm is about 60 and 65% at the maximum by volume. Further, the insulated electric wire has wide fluctuations in the expansion ratio of the insulating layer and the outer diameter of the insulated wire, giving rise to spreads in the signal transmission delay time (.tau.), i.e., .+-.0.1 (ns/m), and characteristic impedance (Z.sub.0), i.e., .+-.10(.OMEGA.).
A method of manufacturing an electric wire insulated with a foamed plastic, which facilitates decreasing the thickness of the insulating layer and also assists in improving the expansion ratio, is disclosed in Published Unexamined Japanese Patent Application No. 3-97746. In this method, 0.01 to 1 part by volume of a carbon halide or a hydrocarbon halide having a boiling point of at least 0.degree. C., which is used as a foaming agent, is added to 1 part by volume of tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA) having a melt flow rate of at most 10 g/10 min. The resin composition thus prepared is extruded to cover a conductor wire, followed by foaming the extruded resin composition layer so as to obtain a desired insulated electric wire. This method makes it possible to obtain an insulated electric wire having such a high expansion ratio as at least 70% and such a small thickness of the insulating layer as at most 0.5 mm.
However, the PFA resin used in the above method is not satisfactory in its flow characteristics, that is, its melt flow rate in manufacturing particularly an electric wire insulated with a foamed plastic which has a small outer diameter and a small thickness of an insulating layer, in which a conductor wire has a diameter of at most 0.2 mm and the insulated wire including the insulating layer has an outer diameter of 10 at most 0.6 mm. Specifically, the pressure in the die portion of the extruder is excessively elevated in the extruding step of the resin of PFA because the melt flow rate of the PFA resin is not adequate. It follows that the smoothness of the insulating layer surface is impaired in the manufactured insulated electric wire. Also, the manufactured wire becomes breakable.
Used in the above method as a foaming agent is a carbon halide or hydrocarbon halide having a molecular weight of 66.1 to 287.2 and containing fluorine, chlorine or bromine. To be more specific, the foaming agent used in this method includes, for example, a methane derivative, an ethane derivative, an ethylene derivative and a cyclic compound. In an Example disclosed in this application, trichlorotrifluoroethane (Fron 113) used as a foaming agent is injected into a molten PFA within an extruder by a pump. In this case, the foaming agent, which contains chlorine etc., gives rise to an environmental pollution problem, as pointed out previously. In addition, the foaming agent is decomposed within the extruder because the molten fluororesin such as PFA resin within the extruder has such a high temperature as at least 300.degree. C., giving rise to discoloration of the plastic obtained.
Further, a fluororesin such as PFA is known as having an excellent outer appearance when extruded only in a restricted shearing region. Thus, where a diameter of the die in the extruder is diminished in manufacturing an electric wire with a foamed plastic which has a small outer diameter and a small thickness of an insulating layer by the method disclosed in the Japanese Patent document referred to previously, it is necessary to decrease the wire coating speed and output of the resin. It is also necessary to decrease the injection amount of the foaming agent into the molten resin. Suppose a foaming agent of monochloro difluoro methane (CHClF.sub.2 : Fron 22) is injected into a molten PFA having a melt flow rate of at most 10 g/10 min in manufacturing a thin insulated electric wire having an outer diameter of at most 0.75 mm, it is necessary to set the injection amount of the foaming agent not to exceed about 0.005 ml/min in order to ensure a sufficient bonding strength between the conductor and the foamed insulating layer. What should be noted is that the injection amount of the foaming agent noted above is close to the lower limit of the flow rate of a precision pump used for the pumping of the foaming agent. Naturally, it is very difficult to control accurately the injection amount of the foaming agent, resulting in failure to obtain an insulated electric wire having a good appearance.